1. Field of the Invention
The present invention relates to canisters for storing, transporting and/or disposing of spent nuclear fuel and, more particularly, to an improved closure mechanism for such canisters, and a method of ensuring leaktight closure of such canisters.
2. Related Art
Spent nuclear fuel is placed into canisters for storage and transportation, and in some instances, for permanent disposal in a geologic repository. As shown in FIG. 1, a typical canister, generally denoted 10, comprises a right-circular cylinder 12 with a bottom plate 14. As can best be seen in FIG. 2, the canister closure at the top of the cylinder 12 includes an outer lid 16 and an inner lid 18, with the canister 10 further including a shield plug 20 for the spent nuclear fuel, denoted SF. The two lids 16 and 18 are field welded, by welds indicated at 22 and 24, after the spent fuel SF and shield plug 20 are installed in the canister 10. It is noted that the spent fuel requirements of Title 10 of the Code of Federal Regulations, Part 72(10 C.F.R. xc2xa772) requires redundant seals such as two welded lids. It should also be pointed out that some conventional canister designs weld the shield plug directly to the canister in order to eliminate the need for an inner lid.
ANSI N14.5, American National Standard for Radioactive Materials-Leakage Tests on Packages for Shipment, specifies the methods for demonstrating that Type B radioactive material transportation packages comply with the containment requirements of 10 C.F.R. xc2xa771. ANSI N14.5 is also the standard applied to spent fuel storage systems and defines the word xe2x80x9cleaktightxe2x80x9d as a leakage rate no greater than 1xc3x9710xe2x88x927 standard cubic centimeter per second (std cm3/s). If it can be demonstrated that a package is leaktight, the package can be stored or shipped without consideration of the package contents. On the other hand, leak rates which are greater than leaktight have to be evaluated against the contents of the package to demonstrate acceptability. ANSI 14.5 also identifies the sensitivity range for a helium mass spectrometer xe2x80x9csnifferxe2x80x9d test as 1xc3x9710xe2x88x923 to 1xc3x9710xe2x88x926 std cm3/s and the sensitivity range for a helium mass spectrometer xe2x80x9cenvelopexe2x80x9d test as 1xc3x9710xe2x88x923 std cm3/s to 1xc3x9710xe2x88x928 std cm3/s.
The typical fuel canister is leak tested using the helium mass spectrometer sniffer test. In this test, after the inner lid is welded to the canister, the canister is filled with helium and the weld joint is tested. The actual sniffer test simply consists of using a probe which is connected to a mass spectrometer and which is held near the weld to sample the ambient air for helium. Once the inner lid penetration is sealed, the void between the inner and outer lids is filled with helium and the sniffer leak test is repeated for the outer lid in the same manner. Such testing indicates that a typical canister has a leak rate of no greater than about 1xc3x9710xe2x88x925 std cm3/s.
In accordance with one aspect of the invention, a canister is provided for storing, transporting, or disposing of spent nuclear fuel, the canister comprising a canister shell, a top shield plug disposed within the canister, and a leak-tight closure arrangement, the closure arrangement comprising: a shear ring forming a containment boundary of the canister, and weld means for welding the shear ring to the canister shell and to the top shield plug.
Advantageously, the shear ring comprises a plurality of pieces welded together, although the shear ring can also be of one piece construction.
Preferably, an outer seal plate is disposed above said shear ring and welded to the shield plug and the canister.
Advantageously, the shield plug includes bolt holes, drilled in an outer surface thereof, for attaching a lifter thereto.
In a beneficial implementation, the mating surfaces of the shear ring and the canister shell are tapered.
In accordance with a further aspect of the invention, a method is provided for producing a leaktight closure for a canister comprising a canister shell and a top shield plug, the method comprising: welding a shear ring to the canister shell and to the top shield plug, supplying a test gas to the canister, welding an outer seal plate to the canister so as to seal the canister and create a space between the seal plate and the shield plug, sampling the air between the shield plug and the seal plate to test internal sealing of the canister, supplying a test gas to the space between the seal plate and shield plug, and testing the outer seal plate for leakage.
Supplying a test gas to the canister preferably comprises removing a pipe plug in the canister, filling the canister with helium and reinstalling the pipe plug after the filling step.
In an advantageous implementation, a leak test adapter is installed on the seal plate after welding of the seal plate and a mass spectrometer is connected to the adapter to sample the air between the shield plug and the seal plate. Preferably, supplying of the test gas to the space between seal plate and shield plug comprises filling the space with helium, and the method further comprises removing the leak test adapter, and using a seal plug to seal the outer seal plate.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.